1. Field of the Invention
This invention relates to a rare-earth bonded magnet used widely for industrial products such as automobiles, business machines, domestic electrification machines and sounder machines, and to a material and a method suitable for manufacturing the rare-earth bonded magnet.
2. Description of the Prior Art
Heretofore, Alnico magnet and Ferrite magnets have been widely used as permanent magnets. However, rare-earth magnets have been developed having excellent magnetic properties as compared to the aforementioned magnets, and the application and demand of rare-earth magnets has remarkably increased in recent years.
The rare-earth magnets contain active metals and are easily oxidized. Therefore, rare-earth magnets of this kind are inferior in their corrosion resistance and heat-resisting properties, especially in an air atmosphere at a temperature higher than room temperature.
Among the rare-earth magnets, R--Fe--B magnets and R--Fe--N magnets contain Fe(iron) as a main element in addition to R (rare-earth metals), and are oxidized to a considerably greater extent as compared to Sm--Co magnets. Accordingly, the R--Fe rare-earth magnets have excellent magnetic properties, however they are disadvantageous in view of their poor oxidation resistance, corrosion resistance, temperature characteristics and heat resistance at temperatures higher than room temperature.
Among the rare-earth magnets, the sintered magnet is densified by sintering. Therefore, it is possible to considerably improve the heat resistance of the sintered magnet by coating the surface of the magnet with, for example, Ni, or resin at the final stage of the magnet manufacturing process. Among the bonded magnets, especially in a magnet manufactured by injection molding using a thermoplastic resin such as polyamide resin, it is possible to improve heat resistance by coating the surface of the magnet in a manner similar to that of a sintered magnet. This is because the surface of magnetic powder is covered completely with the resin.
On the other hand, among the bonded magnets, in a magnet manufactured by compression molding using a binder such as a thermosetting resin (for example, epoxy resin), metals or the like, a large number of vacancies exist between the powdered magnetic material and the binder. Therefore, it is not possible to protect the magnet from oxidation due to the internal vacancies even if the surface of the magnet is completely coated, and oxidation of the magnetic material is unavoidable through the coating layer and the internal vacancies. Consequently, secular change of magnetic properties at room temperature and at temperatures higher than room temperature disadvantageously becomes large and the heat resistance of the magnet is degraded with the passage of time.